Most of the heat energy required for cement production is consumed in the clinker burning process. Thus, if the burning temperature of clinker can be lowered, then it will be possible to reduce energy cost significantly and CO2 emission quantity.
Conventionally, clinker of ordinary Portland cement is produced by burning mixed raw material at 1450° C. or higher in a rotary kiln. The following methods have been considered to lower the burning temperature in the clinker burning step:    (a) Increasing the amount of liquid phase during burning by changing the composition of clinker and facilitating clinker to be generated even at a relatively low temperature (Non-Patent Document 1).    (b) Placing a rapid heating furnace at a previous process of a rotary kiln in order to rapidly heat the input raw material to a melt-reaction temperature or higher, and then performing a low-temperature burning at a temperature ranging from 1300 to 1400° C. in the rotary kiln (Patent Document 1).    (c) Since a high temperature is required for generating alite, which is the main mineral of clinker, mixing beforehand a substance which serves as a nucleus of crystal formation of alite into the raw material of clinker (Patent Document 2).
However, with respect to method (a), the decrease in liquidity and decrease in strength of cement due to the change of mineral composition is a concern. Method (b) may cause an increase in the cost due to modification of the production equipment. On the other hand, method (c) is advantageous in that high-quality clinker can be burned at a smaller heat consumption rate than before, however, method (c) requires a substance having a melting point which is higher than the liquid phase generation temperature (1200 to 1300° C.) of cement clinker, serving as a nucleus of crystal formation of alite.    (d) Lowering the burning temperature of clinker by adding flux (mineralizer) such as fluorite (calcium fluoride) to the raw material of clinker has been studied. For example, by adding a fluorine source and sulfur source as mineralizer to produce ordinary Portland cement, the burning temperature can be lowered by about 100° C.
For example, fluorite or waste which contains fluorine as a fluorine source of mineralizer, and anhydrous gypsum or fuel having a high sulfur content as a sulfur source are used. These are crushed by a crusher, and then mixed with clay, coal ash, or various incinerated ash which serves as an aluminum source of clinker to adjust the clinker consumption rate, the resultant mixture is dried by a drier, thereafter, mixed with dry clay, limestone, silicastone, and iron raw material serving as clinker and the raw material ratio is adjusted, and then the resultant mixture is crushed by a mill to be introduced to a burning kiln.